Vayikra 15:30, "And the kohen shall make one into a sin offering and one into a burnt offering, and the kohen shall effect atonement for her, before the Lord, from the uncleanness of her discharge."
Although the question originally conflated a zavah with woman who is niddah, this obligation applies to a woman who is experiencing discharge beyond her regular menstrual cycle. Regarding the offering brought by a Zav, the male counterpart of a Zavah, the Ibn Ezra on Lev. 15:15 explains that an offering is brought because such discharges are divine punishment for sin. The Ramban adds that the offering expiates the sin to avoid recurrence.